As an adjective sicker
is (
sick) or
sicker can be (obsolete|outside|dialects) certain.
As an adverb sicker
is (obsolete|outside|dialects) certainly.
As a verb sicker
is (mining|uk|dialect) to percolate, trickle, or ooze, as water through a crack.
As a noun sickler is
one who uses a sickle; a sickleman; a reaper.